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tons per year) and kra lignin (<100 000 tons per year). In addition, the advent of hydrolysis and steam-explosion lignin have created new types of technical lignin. 7,8 The use of ionic- liquids or supercritical solvents have furthermore yielded the products ionosolv lignin and aquasolv lignin, respectively, with new and interesting features. 9,10 Lignin is polyaromatic and due to this structure, it is less hydrophilic than polysaccharidic biopolymers, e.g. , cellulose, hemicellulose, starch, alginate or chitosan. 11 It is hence a promising candidate in various applications, including: (i) reduction of wettability of hydrophilic materials, (ii) addition of functionalities, such as protection from UV light, antioxidant and antimicrobial properties, and (iii) tailoring of materials and formulations, e.g. , for controlled substance release, adsorption, or antifouling mechanisms. 12 – 16 However, chemical modi ca- tion is required for most applications of lignin. Such modi - cations frequently make use of lignin's hydroxyl groups, for example, by gra ing reactions during phosphorylation, sulfo- methylation, esteri cation, or amination. 17 The aromatic moieties in lignin can furthermore be targeted for, e.g. , replac- ing phenol in formaldehyde resins. 18 At last, the carboxyl groups in lignin may also serve as reactive sites for polyesters. 19 Interest has also been strong for the use of technical lignin in polymeric materials, e.g. , for thermoplastics or thermosets. 20 Processability of lignin in thermoplastics can be done without modi cation, as lignin is an inherently thermoplastic mate- rial. 21,22 Lignin's glass transition temperature can range from about 60 – 190 °C and may depend on many factors, including the botanical origin and pulping type, moisture content, and chemical modi cation. 23,24 Lignin can also be chemically modi ed to improve the application of lignin as specialty chemicals or in polymeric materials. 25 – 27 Additionally, the utilization of lignin as macromonomer, i.e. , thermoset precursor, can be done as part of polyurethanes, polyesters, epoxide resins, and phenolic resins. 11 End-uses include the production of rigid or elastic foams, rigid and self-healing materials, adhesives, biocomposites, and coatings. 19,28 – 33
One long-held belief is that lignin provides water-proo ng in the wood cell wall to support water-transport. 34 Despite yielding a contact angle below 90°, which would be required to pose as a hydrophobic material, various researchers have shown that lignin can reduce the wettability and water-uptake of wood and pulp products. 18,35 – 37 Hence, both technical and chemically modi ed lignin have been proposed as additives for packaging materials. 38 Reduction of wettability of ber-based packing is a particularly interesting application, considering environ- mental and societal drivers regarding reduction of single-use plastics and environmental pollution. Lignin could thus form the basis of coatings or impregnation blends, provided that the lignin-coating complies with food contact requirements. One example for lignin-blends is the combination with starch during surface-sizing of paper, which can improve extensibility and reduce wetting of the starch-matrix. 35,39 Layer-by-layer assembly with multivalent cations or polycationic polymers has also been done, which can improve the strength and hydrophobicity of cellulose. 40,41 Other applications of lignin, its derivatives and mixtures include the use for controlled-release fertilizers, antifouling membranes, re retardancy, dye sorption, wastewater treat- ment, and corrosion inhibitors. 14 – 16,42 – 44 One publication even reported an unintentional but yet advantageous coating of coir bers, where the lignin delayed oxidation and thermal degra- dation of the bers in a polypropylene composite. 45 Major drivers for using lignin are economical aspects by attributing value to a by-product from pulping or biore nery operations, and sustainability by replacing fossil-based mate- rials with biopolymers. Many applications can thus bene t from the inclusion of lignin in functional surfaces, lms, and coat- ings. The mechanism of action and application mode can hereby di ff er greatly. This review therefore represents an e ff ort to structure and summarize recent progress, where emphasis is put on both the process and nal use for lignin in surfaces and coatings.
2. Fundamentals 2.1. Structure and composition of natural lignin
Lignin is part of the lignin-carbohydrate complexes (LCC) that are found in cell walls of plants and woody materials, as illus- trated in Fig. 1. The cellulose bers are tightly bound to a complex network of hemicellulose and lignin, and the three biopolymers provide strength and stability to the cell walls. In addition to providing structural integrity, lignin helps building hydrophobic surfaces which are important in transport chan- nels for water and nutrients. 46 The complex lignin network consists of the three 4-hydrox- yphenyl propylene units, or monolignols, formed from the parent compounds p -coumaryl- ( p -hydroxyphenyl, H-unit), coniferyl- (guaiacyl, G-unit) and sinapyl alcohol (syringyl, S- unit), see Fig. 2. 47 The monolignols di ff er only in the presence or absence of one or two aromatic methoxy groups ortho to the hydroxyl group. These are synthesized invivo from the aromatic amino acid phenylalanine, formed in the shikimic acid pathway in plants. 48 The resultant monolignols undergo a variety of
Dr Gary Chinga Carrasco was born in Chile and moved to Nor- way in 1987. He graduated with a Cand. scient. degree in cell biology (1997) and Dr ing in chemical engineering (2002). He was one of two recipients of the Norwegian Wood Processing Association Award 2019 for nanocellulose research and winner of the 2021 – TAPPI's International Nanotechnology Division Mid-Career Award. He is
Associate Editor of the Bioengineering Journal, and Editor-in-Chief of the Section – Nanotechnology Applications in Bioengineering. Currently, he is lead scientist at RISE PFI in the Biopolymers and Biocomposites area.
12530 | RSCAdv. , 2023, 13 , 12529 – 12553
© 2023 The Author(s). Published by the Royal Society of Chemistry
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